1. Field of the Invention
The present invention relates to an appearance inspection apparatus which captures an image of a pattern formed on a sample such as a wafer, and which detects a defect by processing the captured image signal and, more particularly, to the configuration of the illumination optics and imaging optics used therein.
2. Description of the Related Art
In the fabrication of semiconductor wafers, semiconductor memory photomasks, liquid crystal panels, and the like, repetitive patterns are formed by repeating a prescribed pattern. It is therefore practiced to detect a pattern defect by capturing an optical image of such patterns and by comparing adjacent patterns with each other. As a result of the comparison, if there is no difference between the two patterns, it is determined that the patterns are free from defects, but if there is a difference between them, it is determined that there is a defect in one of the patterns. The following description will be given by taking as an example a semiconductor wafer appearance inspection apparatus which inspects defects in the patterns formed on a semiconductor wafer. However, the present invention is not limited to this particular application, but can also be applied to an appearance inspection apparatus for inspecting defects on a semiconductor memory photomask, a liquid crystal display panel, or the like.
FIG. 1 is a diagram schematically showing the configuration of the semiconductor wafer appearance inspection apparatus. As shown in FIG. 1, the semiconductor wafer appearance inspection apparatus comprises: a stage 1 for holding a semiconductor wafer 2 thereon; an objective lens 3 for projecting an optical image of the surface of the semiconductor wafer 2; an imaging device (image sensor) 4 for converting the projected optical image of the surface of the semiconductor wafer 2 into an electrical image signal; lenses 21 and 22 for projecting the image of the semiconductor wafer 2, projected through the objective lens 3, onto the image sensor 4; an image signal processing circuit 5 for processing the analog image signal output from the image sensor 4 and converting it into multi-valued digital image data; a digital image data processing circuit 6 for processing the digital image data and detecting a defect by comparing corresponding portions between patterns; and an image data memory 7 for storing the image data for the data processing. An illumination optical system for illuminating the surface of the semiconductor wafer 2 comprises a light source 11 and a half mirror (beam splitter) 15 which is placed in the projection light path between the objective lens 3 and illumination lenses 12 and 13.
A TV camera or the like that uses a two-dimensional CCD device may be used as the imaging device 4, but a line sensor such as a one-dimensional CCD is often used in order to obtain a high-definition image signal; in that case, the stage 1 is moved (by scanning) relative to the semiconductor wafer 2, and the image signal processing circuit 5 acquires the image by capturing the signal of the line sensor 4 in synchronism with the drive pulse signal applied to drive the stage 1.
The illumination optical system used in the semiconductor wafer appearance inspection apparatus will be described below. An illumination optical system for a metallographic microscope is used in the semiconductor wafer appearance inspection apparatus. For the illumination optical system of the metallographic microscope, a bright-field illumination system such as shown in FIG. 2A and a dark-field illumination system such as shown in FIG. 2B are known. In the bright-field illumination system, illumination light from a light source 41 is directed through a lens 42, an aperture stop 43, a lens 44, a field stop 45, and a lens 46 to a half mirror 47 provided in the projection path; then, the light is reflected by the half mirror 47 toward the objective lens 3, and the light passed through the objective lens 3 illuminates the surface of the sample (wafer) 2. The lens 42 projects the image of the light source 41 to the position of the aperture stop 43, and the lenses 44 and 46 project that image to the position indicated by reference numeral 48. This position is the pupil position of the objective lens 3; the illumination light projected to this point illuminates the surface of the wafer 2 with uniform light free from unevenness in light distribution. In the bright-field illumination system, the surface of the wafer 2 is illuminated from the perpendicular direction containing the optical axis of the objective lens, and an image of its specularly reflected light is captured.
On the other hand, in the dark-field illumination system, illumination light from a light source 51 is converted into an annular beam of light by blocking its center portion, as shown in FIG. 2B, and the annular beam of light is then converted into a substantially parallel beam of light through a lens 52. This annular parallel beam of light is projected onto a ring mirror 53 which reflects the beam of light in a direction parallel to the optical axis of the objective lens 3. The ring mirror 53 is a ring-shaped (or more specifically, ellipsoidal ring-shaped) reflective mirror which allows light to pass through its center portion centered around the optical axis of the objective lens 3 but reflects light at its outer peripheral portion. The annular illumination light reflected by the ring mirror 53 is focused through a ring-shaped condenser lens 54 and illuminates the portion of the wafer 2 near the optical axis of the objective lens 3.
As illumination light specularly reflected at the surface of the wafer 2 is not captured, the dark-field illumination system has the advantage of increasing the relative signal strength of the diffracted light occurring due to the presence of a defect (a short circuit) on the wafer 2; in recent years, the need to inspect the patterns on wafers under dark-field illumination has been rapidly increasing because of ever decreasing pattern feature size.
As described above, the bright-field illumination system and the dark-field illumination system have their own advantages, and appearance inspection apparatuses have been designed that have both the bright-field illumination system and the dark-field illumination system and that are capable of switching between these two systems.
For example, Japanese Unexamined Patent Publication No. 2003-149169 discloses an appearance inspection apparatus comprising a bright-field illumination system for providing illumination light that covers the range containing the optical axis of the objective lens, and a dark-field illumination system for providing illumination light that covers a range outside the region centered around the optical axis of the objective lens by using a reflective mirror mounted in a position outside the projection path of the objective lens.
On the other hand, Japanese Unexamined Patent Publication No. 2004-101406 discloses an appearance inspection apparatus that achieves both bright-field illumination and dark-field illumination by providing circular filters complementary to each other, one disposed at a conjugate point 14 to the pupil plane 8 of the objective lens provided in the illumination optical system implementing the bright-field illumination system and the other at a conjugate point 23 to the pupil plane 8 of the objective lens provided in the imaging optical system.
Likewise, Japanese Unexamined Patent Publication No. 2004-101403 discloses an appearance inspection apparatus that comprises a circular filter provided at a conjugate point 14 to the pupil plane 8 of the objective lens provided in the illumination optical system implementing the bright-field illumination system, a polarization mirror as a beam splitter 15, and a member containing a half-wave plate provided between the objective lens 3 and the polarization mirror, and that achieves an effect equivalent to that of a dark-field illumination system by blocking low-order diffracted light from the sample under inspection.
Next, the projection optical system (imaging optical system) used in the appearance inspection apparatus will be described. In the appearance inspection apparatus, the lenses 21 and 22 are arranged as imaging optics for projecting the image of the sample 2, projected through the objective lens 3, at a desired magnification onto the image sensor 4. Here, an image of the frequency component intensity distribution of the image of the sample 2 under observation is formed in the pupil plane 8 of the objective lens 3; accordingly, by providing a spatial filter (spatial frequency filter) at the pupil plane 8 or at the conjugate point 23 to the pupil plane 8 in the imaging optical system, when the image under observation is an image of a pattern having periodicity such as a pattern in a memory cell area, for example, the frequency components corresponding to the pattern can be masked and the relative signal strength of diffracted light occurring from other portions, i.e., defective (short circuited) portions, can be increased.
For example, U.S. Pat. No. 6,686,602 discloses an appearance inspection apparatus in which a programmable spatial filter is provided at the Fourier plane of the objective lens 3.